Congestive heart failure occurs when cardiac function is impaired to the point that there is insufficient blood flow to support a person's activities. It is the largest problem in cardiac care today and there are more than 5 million patients with this diagnosis in the U.S. In general, patients can be treated with medications, but when symptoms are advanced, cardiac transplantation or the implantation of blood pumps may be necessary. Unfortunately, cardiac transplantation is limited by the donor pool and each year fewer than 2,000 patients undergo this procedure.
It has also been determined that the inflow cannula for a blood pump may result in clinical compositions. Clots may form around the inflow cannula as it sits in the chamber of the left atrium (or left ventricle). These clots may break off and enter the pump and then pass anywhere in the circulation. Embolization of material can cause damage to any part of the body but embolization to the brain is the most feared complication. Entry of clot into the brain generally causes brain dysfunction and the results can be mild—such as a temporary and slight slurring of speech or gait impairment to a terrible catastrophic loss of function that leaves a patient in a wheelchair or bedridden and unable to communicate. In this situation death may be a potential outcome.
Another deficiency that is encountered with cannulation of the left atrium is collapse of the left atrium and occlusion (temporary or permanent) of the inflow cannula by atrial tissue. This is similar to what happens with a vacuum cleaner tube that is brought close to material such as a curtain—the tube becomes obstructed and eventually occluded by the fabric as it is progressively sucked inside the tube. The left atrium is a very pliable thin walled structure (usually 1 mm to 3 mm in thickness). An atrial cannula is fastened to the atrial septum with 5 mm to 10 mm or more of cannula tip projecting into the left atrium. A cannula tip sitting in the left atrium and flowing at 2 to 3 liters per minute can easily suction a part of the atrial wall into the cannula leading to cannula obstruction and loss of pump flow. As flow stops in the cannula, clots may form in the cannula and when the obstruction is relieved a clot may be ejected into the circulation. Alternatively, the cannula may completely clot off and make it impossible to pump blood when the obstruction is relieved. The atrial tissue may also be injured. The first patients treated with superficial supplemental blood pumping had relatively low levels of flow, but new generations of pumps can flow at much higher rates and it would be useful to have a way to avoid the problem of a cannula suctioning atrial tissue that results in cannula obstruction and clotting and still allow the pump to flow at rates of 5 liters per minute or more.
Another problem with a cannula implanted in the left atrium is that it must be fastened securely in position. When a cannula is directed into the left atrium from the chest, it is necessary to seal the cannula to prevent bleeding and dislodgement. Typically this requires the placement of a suture circumferentially around the cannula to hold it in position e.g., a purse string suture. Also, there is frequently tissue such as fat around the entry site, and this must be dissected to ensure that no fat is forced into the heart when the cannula is pushed into the heart. If fat is pushed into the heart, it can embolize anywhere in the circulation. This means the surgeon must make a chest incision big enough to pass sutures and gain good exposure of this region. A better attachment to the heart would allow the surgeon to reduce the size of his incision and even work through endoscopic ports. Alternatively, the cannula may be passed into the left atrium from the right atrium, the hole through the septum must be sealed and the cannula must be held in place. Sealing and holding the cannula in place is not simple. An improved method of holding a cannula in place that eliminates the need for suturing and provides a secure and fluid tight seal would be valuable to patients and surgeons in preventing complications from superficial blood pumping.
Furthermore, creating a defect in the wall of the atrium to introduce a cannula is also a difficult problem. Simply pushing a cannula through a small hole is dangerous since the atrial tissue may tear. This may leave a large hole which is difficult to close.
Introducing a cannula inside the heart is usually done by placing the cannula inside catheters for delivery. This increases the diameter of the delivery system. It also creates further problems because a cannula may “snag” inside the delivery catheter. To guarantee delivery, there are serious development and manufacturing problems in maintaining the correct gap tolerances between the cannula and the catheter. Lubricants may be necessary. And even with excellent design, the operator may be forced to insert a cannula along a circuitous course. Bends may make it impossible for even the best engineered system to permit delivery of the cannula. It would be useful to eliminate the need for a catheter
In summary, superficial blood pumping is a promising therapy for many patients with heart failure and it is now technically possible to pump the entire blood flow from a surface location with a very small pump. Improvements in methods and devices for cannulating the heart will make this procedure easier and safer.